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  1. #1
    Dad
    Reputation: Smudgemo's Avatar
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    Tandem Kid Crank Q

    I've got a Comotion tandem and a kid who isn't quite long enough to reach the pedals or use the stoker cranks. I'd like to save myself $70 by building a kid-crank setup, but would like some advice before I start.

    I've got a 68mm BB shell and I'll probably use the cranks off of her soon-to-be-stripped 20" Giant, but I'm unsure what kind of clearance I should have for the tube acting as a clamp on the seat tube. It's in storage right now so I don't have the measurements handy, but to complicate things, it appears to be an externally butted seat tube.

    The production version of this setup that I've seen looks like its only got a few inches of tubing welded to the BB shell, but I'm thinking of using a longer piece to help avoid damaging the seat tube or paint. I'm planning to braze on four seat binder bosses, then cut the tube in half.

    My questions are:
    1. If the OD of the seat tube is X, what kind of ID should I shoot for on the clamping tube?
    2. How long should the clamping tube be? Three to four inches seems plenty, but...
    3. Any other suggestions for protecting the paint?

    Thanks,
    Ryan

  2. #2
    mtbr member
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    There are crank shorteners avaiable.

    I think LOOK made them for track bikes.

    http://harriscyclery.net/product-lis...ren-t914-qc49/

    or a bolt on BB shell.

    http://harriscyclery.net/product/pro...r8951-qc49.htm

  3. #3
    Ultra Ventanaphile
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    Quote Originally Posted by C Dunlop
    There are crank shorteners avaiable.

    I think LOOK made them for track bikes.

    http://harriscyclery.net/product-lis...ren-t914-qc49/

    We use these and they work well....
    Attached Thumbnails Attached Thumbnails Tandem Kid Crank Q-adrian-aaron-tandem-sm.jpg  

    -Aaron G.

    "Before D.W., "anti-squat" was referred to as pedal feedback."

  4. #4
    Old school BMXer
    Reputation: Blaster1200's Avatar
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    I think the issue is that the child's legs are too short to reach the cranks, which is why the OP wants to mount the bottom bracket to the seat tube.

    Depending on where the butt is of the seat tube, you can get some tubing with an OD 1/8" larger with a wall thickness of .058". This should give you enough clearance with the paint thickness along with a clear, thin adhesive to protect the frame. If the .058" is too tight over the film and paint, try some .049" wall thickness. I'd probably make the clamp about 2-3" long, but don't have any data to support my thought.

    http://www.aircraftspruce.com/catalo...tubing_un1.php

    I'm building a tandem right now, and I've been planning on building a BB temporary mount for it as well.

    If you want a bolt-on solution, go about half-way down this page and check out the links under Child Stoker Bottom Bracket Shells. They offer some with nylon shims to protect the paint.
    http://www.precisiontandems.com/catframepart.htm
    May the air be filled with tires!

  5. #5
    Dad
    Reputation: Smudgemo's Avatar
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    That's right, it's about mounting the BB not crank length. I'll simply flip some cranks from a kid's bike for now, and we'll be good there. The nylon shim looks like a good bet, but I was also thinking of lining the clamp with some of that stick-on plastic that protects lower panels of cars from chipping.

    I was mostly hoping to confirm that I was on the right track. I've got to order some more tubing from Wicks, so I'll probably buy a foot in a couple of wall thicknesses and play around with it. That link to the bolt-on piece is exactly what I was referencing, but I like the idea of a longer clamping tube far more. I came across a photo of someone's Santana with a kinked tube from a botched install, and the thought of the repair cost still makes me cringe.

    Thanks

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